纳米材料和纳米结构第五讲.ppt

Based on inorganic polymerization reactions（无机聚合反应） Used to prepare inorganic oxides, such as glasses or ceramics, especially multicomponent systems（多组分体系） Be able to control the structure on nanoscale from the earliest stages of processing Easy to obtain nanostructured grains with compositional homogeneity（组分均匀性）, higher purity, lower processing temperatures over both conventional ceramic processing and traditional glass melting by controlling the chemical additives（添加剂） and processing, but without adopting vacuum conditions Based on the difference of the precursors used, the sol-gel processing is roughly divided into three groups: Silica （二氧化硅）sol-gel processing Metal alkoxide （金属醇盐）processing Pechini-type processing The shape and size of polymeric（聚合的）structural units are determined by the relative values of the rate constants for hydrolysis (kH) and polycondensation reactions (kC) To reduce the particle size, kH should be larger than KC Fast hydrolysis and slow condensation favor formation of linear polymers Slow hydrolysis and fast condensation lead to larger, bulkier, and more ramified（网眼状的） polymers As the sol particles grow and collide, condensation occurs and macroparticles begin to form; The sol becomes a gel when it can support a stress elastically. The change is gradual as more and more particles become interconnected and this is typically defined as gelation（凝胶化） point or gelation time tg; All subsequent stages of processing depend on the initial structure of the wet gel formed in the reaction bath during gelation; Polymerization reactions are usually thermally activated. In view of the dependence of tg on solution pH, the gelation can be either acid or base catalyzed. The activation barrier to aggregation increases linearly with the size of two equal particles. Thus the rate of aggregation would decrease exponentially with their size. Smaller particles will aggregate with larger ones at a much higher rate. Thus two distrib